Ion Chromatography - Dispersive Interactions

Dispersive Interactions

Dispersive interactions are the most difficult to describe probably because they are still not clearly understood. There are a number of definitions in the literature but the best is probably that given by Glasstone many years ago but is still directly applicable today.

Quoting Glasstone,

"although the physical significance probably cannot be clearly defined, it may be imagined that an instantaneous picture of a molecule would show various arrangements of nuclei and electrons having dipole moments. These rapidly varying dipoles when averaged over a large number of configurations would give a resultant of zero. However, at any instant they would offer electrical interactions with another molecule resulting in interactive forces".

Dispersive interactions are diagramatically depicted in figure1. In these diagrams the color of the spots indicates the nature of their charge. The red and black dots represent the rapidly varying dipoles viewed at any instant that can interact with a similar pattern of dipoles situated at the same instant on another molecules.

Figure 1. A Diagrammatic Representation of Dispersive Interaction between a Solute Molecule and a Phase Molecule The strength of the dispersive interactions of a molecule have been shown to be proportional to its' polarizability. Note, the term polarizability is an electrical property calculated from its dielectric constant, it does not imply that a dispersive substance is polarizable, which means that a dipole can be induced into the molecule by thc close proximity of a molecule having a permanent dipole. In fact, aliphatic hydrocarbons that are typically dispersive in character are not polarizable but they do posess a plarizability Dispersive interactions are the only type of molecular interaction that can take place exclusively and in the absence of any other type of interaction. Conversely. polar interactions and ionic interactions must always be accompanied by dispersive interactions and possibly polar interactions, which will add to the overall magnitude of the net interacting force.

Polar Interactions

Polar interactions arise from electrical dipoles that may occur as permanent or induced dipoles. As stated above, polar interactions cannot occur in isolation, but will always be accompanied by dispersive interactions and in certain circumstances may be combined with ionic interactions. Polar interactions can involve very strong molecular forces that approach in energy to that of a weak chemical bond. An examples of such strong polar forces is in 'hydrogen bonding', e.g. in the association of water with itself.